Ink jet printers have rapidly achieved a wide commercial success due to a combination of features, principally low cost, rapid printing speed capability, quiet operation and excellent graphic capability. Jet printing is a non-impact method for forming patterns, which include both typography and graphics in response to computer-generated electronic signals. The signal causes the printer to produce droplets of ink that are project on to a substrate such as paper or plastic film of varying types. A printed image is comprised a multitude of discreet dots laid in an orthogonal pattern. The sharpness of the image in part depends on the size of the dots which in turn are a function, inter alia, of the print head aperture size, and the media (nature and quality of the paper or plastic film substrate, including surface characteristics). The composition and character of the inks also plays a vital part in the image quality, light fastness, resolution, water and smear resistance, and storage stability, among others.
The term ink jet as used herein includes a variety of methods of forming and projecting the dots, including vaporization (so-called bubble jet technology) and hydraulic droplet projection. While first commercial ink jet printers laid down from 1 to 200 dots per inch, they now exceed 700 dpi. Printer inks for ink jet printers are ordinarily of three basic types: pigment-type inks, dye-based inks and toned pigment inks, which employ a combination of dye and pigments. Regardless of the type, they need to have for optimum print head performance and image quality suitable properties of viscosity, stability, drying speed, and surface tension. They also need to retain the water or other solvent in the ink cartridge so that no precipitate or crystals form around the nozzle openings when the printing is interrupted for an extended period of time. Further, the ink viscosity and drying speed should not be so high that the nozzles become clogged by the ink after prolonged nonuse or by clinging to the outer edge of the nozzles causing build-up around the nozzle giving rise to a nozzle port bridge over or irregular edges leading to a ragged dot formation.
Dye-based inks are particularly suitable for ink jet printing because the dye compound is fully soluble in the carrier, typically water or a water/solvent system. However, while dye-based inks are satisfactory for many applications, the dye tends to wick in paper fibers causing the dots to have a feathered edge. The resulting dots do not have the sharp boundaries needed to produce an ultra-high resolution image unless a special media (paper or plastic sheet material) is used. Also, the dyes, unless they become covalently bonded to the media, tend to smear after the printing operation due to water solubility. The contradiction is that the dye compound should be soluble in the water carrier, yet after application and evaporation of the water, the resulting dye should be water fast. These, being contradictory properties, ordinarily results in one property being sacrificed in favor of the other. A good dye-based ink composition is shown in U.S. Pat. No. 5,364,462 to Crystal, et al. of Graphic Utilities, and is particularly directed to improved water solubility and water fastness by use of hydroxyethlated polyethylene imine polymers, and alternate embodiments.
Pigment-type inks, on the other hand, generally produce higher quality images on a wider range of media type and qualities. Properties that are important in this regard include size of the pigment particles, the carrier or vehicle in which the pigment particles are suspended, nonsettling characteristics of the ink composition and the concentration of the pigment in the ink composition. As compared to paints, pigment-based inks are generally of much lower pigment concentration being on the order of 30% as compared to paints which may have a loading on the order of up to 60 to 70%, which in some cases include latex(s) as part of the solids loadings. In part this is due to the nature of the method of application, ink jet printing. Inks with greater loadings exhibit higher viscosity and poor printing qualities due, in part to the fact that there must be a sufficient carrier for the print head to propel the droplet toward the media. Further, high pigment concentrations lead to faster nozzle build-up and plugging. Further, there is a problem with respect to drop out of pigment. When pigment concentrations get too high, they tend to agglomerate due in part to breakdown of the suspension mechanism in the ink composition system. The suspension mechanism can range from purely ionic repulsion to true emulsion formation. However, many factors enter into the stability of emulsions and they can "break" when there occurs a drop on demand (long inactivity between usages abbreviated DOD), for example upon evaporation of carrier/solvent, or chemical interaction of the ink composition components. A typical aqueous pigmented ink is disclosed in U.S. Pat. No. 5,221,334 to Ma et al of DuPont which is particularly directed to the use of block copolymer dispersants. The use of humectants in pigmented inks to resist ink jet nozzle clogging is shown in U.S. Pat. No. 5,106,417 to Hauser, et al of Ciba-Geigy.
Optical density is a measure of the coverage characteristics of an ink. With a given drop volume, optical density is an important indicator of how well the ink forms a crisply defined dot (or other image) with good opacity characteristics so that the light reflected from the printed image is that of the ink, yet not permitting partial show-through or see-through of the underlying media color, be it blank white paper or a previously printed media. Thus, it is an important goal to provide the highest optical density for the lowest pigment concentration so that the ink has good printing characteristics, that is handles well in the ink jet cartridges and produces a high quality image. As noted above, optical density can be improved by an increase in pigment concentration, but at the severe sacrifice of print-handling characteristics.
Optical density is measured by an optical densitometer after printing and letting dry multiple standard test blocks (1/2".times.1") with a commercially available ink jet printer on commercially available copier paper at a defined dot pitch, typically 600.times.600, 600.times.300 or 300.times.300. Reflectance readings are taken by-the densitometer from multiple spaced locations from each test print block, and the optical density (reciprocal of reflectance) is the arithmetic average of the test point values. Crystallization or clogging by ink of orifices is determined by visual examination after an accelerated non-use period in which the cartridge is exposed to air without a cover over the jets for several hours, typically 10-48 hours, and which may be accelerated by heating to 100.degree. F.